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ADE7768ARZ-RL

Energy Metering IC with Integrated Oscillator and Positive Power Accumulation

Analog Devices 

ADE7768

FUNCTIONAL DESCRIPTION

THEORY OF OPERATION

The two ADCs in the ADE7768 digitize the voltage signals from

the current and voltage sensors. These ADCs are 16-bit Σ-Δs

with an oversampling rate of 450 kHz. This analog input

structure greatly simplifies sensor interfacing by providing a

wide dynamic range for direct connection to the sensor and

by simplifying the antialiasing filter design. A high-pass filter

in the current channel removes any dc component from the

current signal. This eliminates any inaccuracies in the real

power calculation due to offsets in the voltage or current

signals.

The real power calculation is derived from the instantaneous

power signal. The instantaneous power signal is generated by

a direct multiplication of the current and voltage signals. To

extract the real power component (the dc component), the

instantaneous power signal is low-pass filtered. Figure 15

illustrates the instantaneous real power signal and shows how

the real power information can be extracted by low-pass

filtering the instantaneous power signal. In the ADE7768,

this signal is compared to 0 and only positive real power is

accumulated for F1, F2, and CF pulse outputs. This scheme

correctly calculates real power for sinusoidal current and

voltage waveforms at all power factors. All signal processing

is carried out in the digital domain for superior stability over

temperature and time.

CH1

ADC

HPF

DIGITAL-TO-

FREQUENCY

F1

F2

CH2

MULTIPLIER

ADC

≥0

LPF

DIGITAL-TO-

FREQUENCY

CF

0Wh ACTIVE ENERGY

INSTANTANEOUS

0W POWER

Figure 16. Positive-Only Energy Accumulation

Power Factor Considerations

The method used to extract the real power information from

the instantaneous power signal (that is, by low-pass filtering) is

still valid even when the voltage and current signals are not in

phase. Figure 17 displays the unity power factor condition and a

displacement power factor (DPF) = 0.5 (a current signal lagging

the voltage by 60°). Assuming that the voltage and current

waveforms are sinusoidal, the real power component of the

instantaneous power signal (that is, the dc term) is given by

⎜⎛ V × I ⎟⎞ × cos (60°)

(1)

⎝2⎠

This is the correct real power calculation.

INSTANTANEOUS

POWER SIGNAL

POWER

INSTANTANEOUS REAL

POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL – p(t)

INSTANTANEOUS REAL

POWER SIGNAL

V× I

2

TIME

TIME

Figure 15. Signal Processing Block Diagram

The low frequency outputs (F1 and F2) are generated by

accumulating positive-only real power information. This low

frequency inherently means a long accumulation time between

output pulses. Consequently, the resulting output frequency is

proportional to the average positive-only real power. This

average positive-only real power information is then accumu-

lated (by a counter) to generate real energy information (see

Figure 16). Conversely, due to its high output frequency and

shorter integration time, the CF output frequency is propor-

tional to the instantaneous positive-only real power. This is

useful for system calibration, which can be done faster under

steady load conditions.

0V

CURRENT

VOLTAGE

POWER INSTANTANEOUS

POWER SIGNAL

INSTANTANEOUS REAL

POWER SIGNAL

TIME

V× I

2

COS (60°)

0V

TIME

VOLTAGE

CURRENT

60°

Figure 17. DC Component of Instantaneous Power Signal Conveys

Real Power Information, PF < 1

Rev. A | Page 10 of 20

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